The invention relates to a deflection yoke for a colour cathode-ray tube.
A deflection yoke includes a pair of vertical deflection coils, a pair of horizontal deflection coils in the form of a saddle and a ring of ferromagnetic material surrounding the deflection coils so as to concentrate the deflection fields in the appropriate region. A cathode-ray tube intended to generate colour images generally includes an electron gun emitting three coplanar electron beams, each beam being intended to excite a phosphor of a defined primary colour (red, green or blue) on the screen of the tube.
The electron beams scan the screen of the tube under the influence of the deflection fields created by the horizontal and vertical deflection coils of the deflection yoke fixed to the neck of the tube.
The three beams generated by the electron gun must always converge on the screen of the tube, otherwise a so-called convergence error is introduced which falsifies, in particular, the rendition of the colours. In order to make the three coplanar beams converge, it is known to use so-called self-converging astigmatic deflection fields; in a self-converging deflection coil, the intensity of the field or the lines of flux which are caused by the horizontal deflection winding are generally in the form of a pin-cushion in the region of a portion of the coil which is located somewhat to the front of the latter on the side facing the screen of the tube.
Moreover, under the action of uniform horizontal and vertical deflection fields, the volume scanned by the electron beams is a pyramid whose apex is coincident with the centre of deflection of the deflection yoke and whose intersection with a non-spherical screen surface exhibits a geometrical defect called pin-cushion distortion. This geometrical distortion of the image increases as the radius of curvature of the screen of the tube increases.
Self-converging deflection yokes generate astigmatic deflection fields which make it possible to modify the North/South and East/West geometry of the image and, in particular, partially correct for the North/South pin-cushion distortion.
The correction of both the convergence of the electron beams and the North/South geometry of the image on the screen, by means of a particular configuration of the conductors making up the deflection coils, has been difficult to achieve without additional components, such as metal pieces or permanent magnets. The additional components are placed so as to cause local modification of the deflection fields. These additional components may be expensive and may lead to overheating problems associated with the operating frequency, particularly when they are used to modify the horizontal deflection field, since the current trend is towards increasing the said frequency up to 32 kHz or even 64 kHz.
Moreover, these image-geometry and convergence problems are associated with the planarity of the screen and increase with the radius of curvature of the said screen. Conventional cathode-ray tubes manufactured a few years ago, using a screen of spherical shape, generally have a radius of curvature R. When the screen has a relatively large radius of curvature, greater than 1R, such as 1.5R or more for example, it becomes increasingly difficult to fix the problems mentioned above merely by means of suitable fields generated by the deflection coils.
Published European Patent Application EP 701,267 discloses a means of controlling the North/South geometry of the image created by the deflection yoke on the screen surface, as well as the convergence of the beams, using a ring or core of ferromagnetic material to concentrate the deflection fields. Part of the core, closest to the screen of the tube, is arranged in such a way that the regions closest to the vertical axis of symmetry have a magnetic reluctance greater than the regions closest to the horizontal axis of symmetry. However, this arrangement may require the use of an additional field shaper for local correction of the defects.
It may be desirable to employ in a deflection yoke for a colour cathode-ray tube a magnetic core having a geometry of which makes it possible to correct both convergence and geometrical errors without using additional field shapers.
In accordance with an inventive feature, a deflection yoke includes a pair of horizontal deflection coils and a pair of vertical deflection coils. An approximately frustoconical ferromagnetic core is placed, at least partly, over the deflection coils. The core has a vertical plane of symmetry, YZ, and a horizontal plane of symmetry, XZ. The magnetic reluctance of the core, in its front part, in the vicinity of the horizontal plane of symmetry, is greater than in the vicinity of the vertical plane of symmetry.